Faraday Rotation Studies of an<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>R</mml:mi></mml:math>-Like Center in Neutron-Irradiated MgO. I. Experimental

Glaze, James A.; Kemp, J. C.

doi:10.1103/physrev.178.1502

Cited by 6 publications

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References 12 publications

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“…AS expected from the results of Glaze and Kemp [ 8 ] the magnetic field dependence of the MCD of the 524 nm line a t x 10 K agreed satisfactorily with that calculated for a spin-only ground state g-value. Taking CSv as the symmetry of an 12-like centre the selection rules for a 2E --f 2A transition are shown in the top half of Fig.…”

supporting

confidence: 81%

“…1, carve b). As expected froni the results of Glaze and Kenip [8], the 524 nni line gave an obvious absorption-like teniperature-dependent MCD signal, be observed in the MCD spectrum on the high-energy side of the 524 nni line; this structure is of vibrational origin and will be discussed later. A weak, broad, teniperat]ure-dependent derivative-type MCD signal near 600 nin was observed and a further weak, broad, temperature-dependent signal was also observed below 500 nm.…”

Section: Trigonal Centres In M G Osupporting

confidence: 50%

“…The 524, 642, and 649 nin lines have been supposed [7] to arise from different charge states of K centres (arrays of three nearest-neighbour anion vacancies lying in (111) planes), though other centres [l] have been suggested. Glaze and Keinp [8] l) Canberra A.C.T. 2600, Australia. observed paramagnetic Faraday rotation for the 524 nm line as expected for a 2E -+ 2A transition, no paramagnetic Faraday rotation from the 642 nm line, as expected for a =A --f IE transition, and did not observe the 640 nni line, interpreted as a 2A --f 2E transition [6] with sufficient intensity to make meaningful Faraday niemureiiients.…”

Section: Introductionmentioning

confidence: 99%

“…Glaze and Kemp [8] deduced that the sign of the spin-orbit splitting in the 2E ground state of the centre responsible for the 534 nni line was positive, by comparison of the sign of the Faraday rotation relative to that of the rotation in the F+ centre in MgO and CaO [ 15, 16J. Such a coniparison however does not take account of the selection rules involved in the transition.…”

mentioning

confidence: 95%

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Magnetic Circular Dichroism of Neutron‐Irradiated MgO and CaO

Vance¹,

Manson²

1977

Physica Status Solidi (b)

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Magnetic circular dichroism (MCD) measurements are made on neutron-irradiated MgO to examine the sharp 524, 642, and 649 nm optical absorption lines, which are known to be A -E transitions within trigonal centres. The MCD of the 524 nm line is strong at low temperatures, as expected from previous Faraday rotation measurements, and vibrational sidebands are observed in the MCD measurements. The MCD signals from the 642 and 649 nm lines are very weak, but temperature-dependent : assuming the centres are paramagnetic the magnitudes of the spin-orbit splittings of the ZE-states are calculated as & 0.1 cm-l from the MCD data. I n neutron-irradiated CaO some sharp, strong temperature-dependent MCD signals are observed after the samples are annealed at moderate temperatures; the corresponding absorption lines a t 465, 573, 602, 672. and 678 nm are not previously reported. The centres responsible for these lines appear to be paramapnctic and to possess a t least axial symmetry.Les mesures de dichroisme magnCtique circulaire (DMC) ont 6th faites sur MgO irradk a u s neutrons pour Btudier les bandes ktroites d'absorption optique 524, 642 et 649 nm qui sont atkribuCes A la transition A 6-E des centres trigonaux. Le DMC de la raie 524 nm est fort It basses tempbratures, comme le prkvoyaient les mesures de rotation Farsday, e t on a observe des bandes latterales vibrationnelles dans les mesures de DMC. Les signaux DMC des raies 642 et 649 nm sont trek faibles, mais dbpendantes de la temperature: en supposant que les centres sont paramagnbtiques, on calcule It partir d'une donnee DMC que les sbparations spin-orbites des Btats E, sont de l'ordre de 0 , l cm-l. Sur le CaO irradib aux neutrons, quelques signaux DMC Ctroits, dependant fortement de la temperature, sont observbs aprks que les Bchantillons eussent 6tb rbduits h teinp6ratures modkrbes; les raies d'absorption correspondantes h 465,573,602.672 et 678 nm n'avaient pas 6t6 signalbes prhedemment. Les centres responsables de ces raies apparaissent Ftre paramagnCtiqnes e t possbder au moins une symetrie axiale.

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supporting

confidence: 81%

Section: Trigonal Centres In M G Osupporting

confidence: 50%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 95%

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Magnetic Circular Dichroism of Neutron‐Irradiated MgO and CaO

Vance¹,

Manson²

1977

Physica Status Solidi (b)

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“…(18) L. S. Singer, 5th Carbon Conference, Vol. II, Journal of the American Chemical Society / 94:19 / September 20,1972 T h i s c o n t e n t i s Similarly, it does not seem reasonable to identify Vi centers with other types of impurities since the arguments used to rule out impurities as active centers can be repeated here, in view of the analogy of behavior between active sites and Vi centers (Table IV). Rather, the fact that Vi centers disappear irreversibly by heating MgO in air at temperatures at which metastable (111) planes disappear to be replaced by planes of lower free energy makes us suspect that (111) surface planes contain the Vi centers.…”

Section: Irreversibly Lostmentioning

confidence: 99%

Activation of hydrogen at 78.deg.K on paramagnetic centers of magnesium oxide

Boudart¹,

Delbouille²,

Derouane³

et al. 1972

J. Am. Chem. Soc.

195

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The HzDz equilibration at 78°K has been studied in a flow system over three varieties of MgO powders with or without addition of impurities. The only impurity that affected the catalytic activity consisted of protons, the surface content of which could be regulated by pretreatment of the samples in vacuo at temperatures between 500 and 900°C. Thus, samples were prepared with catalytic activity differing by more than five orders of magnitude. The activity was correlated with the spin density of an electron-deficient paramagnetic center which was shown to be a surface center. The exchange-narrowed symmetrical epr line, corresponding to this new V-like center called VI, was assigned to a triangular array of 0-ions situated on a (111) plane of MgO. The catalytic site then consists of an OH-ion adjacent to the VI center which adsorbs the deuterium molecule to form a quasi-sorbed Rideal-Eley transition state bringing about exchange. The proposed ion-molecule type reaction explains its low activation energy (2 kcal mol-1) and the possibility of carrying out the reaction at 78°K without the participation of transition metal ions. The apparent necessity of metastable (1 11) planes on MgO for obtaining catalytic sites and VI centers was suggested by the irreversible loss of activity and spin density upon heating MgO samples in the presence of HzO vapor at temperatures at which recrystallization sets in with a change of crystal habit. Thus the catalytic site is associated with a proton impurity, an electronic defect, and a specified crystallographic plane.authors to catalyze the Hz-D2 exchange at 78"K, without the need for any irradiation before or during the reaction. Much interesting work on MgO powders preevacuated at various temperatures and preirradiated with uv light was published subsequently by Lunsford6 and by Harkins, et aZ.2 who studied the Hz-D2 exchange at 195°K and room temperature (which will be abbreviated hereafter: 298"K), respectively. This later work, which also involved epr spectroscopy, led the authors to propose V-type centers as catalytic sites and in particular' to assign to 0ions the role of sites responsible for the H2-D2 reaction at 78°K on MgO preevacuated at 300°C.

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Anion vacancy centers in alkaline earth oxides

Henderson

1980

Critical Reviews in Solid State and Materials Sciences

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Faraday Rotation Studies of anR-Like Center in Neutron-Irradiated MgO. I. Experimental

Cited by 6 publications

References 12 publications

Magnetic Circular Dichroism of Neutron‐Irradiated MgO and CaO

Magnetic Circular Dichroism of Neutron‐Irradiated MgO and CaO

Activation of hydrogen at 78.deg.K on paramagnetic centers of magnesium oxide

Anion vacancy centers in alkaline earth oxides

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